Use the ‘createinstallmedia’ command in Terminal to create your bootable usb installer:

Download macOS Sierra 10.12 Developer Preview from the Mac App Store after redeeming the code from the Apple Developer Download site. Quit the installer if it opens automatically after downloading. The installer will be in your Applications folder.

Mount your USB flash drive or other volume. You could also use a secondary internal partition.

Format/Erase your drive and name it “macOSSierra” if you intend on copying and pasting the below command.